1. Field of the Invention
The invention relates in general to RF excited waveguide lasers and in particular to improvements to RF excited waveguide laser components.
2. Prior Art
In general, RF excited waveguide lasers having a distributed inductance are known. Referring to prior art FIG. 1, a conventional RF laser disclosed in U.S. Pat. No. 4,787,090 ('090) is shown. The '090 patent discloses a distributed inductance RF excited waveguide arrangement which is inserted into a metal housing structure which serves as both the vacuum housing and the structure to support resonator mirrors. The '090 patent teaches clamping the inserted assembly within this housing structure by deforming one surface of the structure with an external clamping plate. It has been found in practice that this clamping approach has a number of problems which adversely effect the laser's integrity and performance. For example, the clamping force is difficult to control which has resulted in clamping forces that are so large that fracture of the internal ceramic waveguide structure has occurred. In addition, this clamping arrangement requires that one surface of the vacuum housing be very thin so that it can be deformed by the clamping plate. This results in a reduction of the stiffness of the housing thereby compromising the optical alignment stability of the laser.
Referring to prior art FIGS. 2 and 3, a conventional folded waveguide which uses a common electrode to excite a gas discharge within a Z-fold optical waveguide structure so that a gas discharge is obtained in all the channels, is shown. The waveguide comprises a ceramic substrate 4 with waveguide channels 6 formed therein. Metal electrodes 8 are placed on either side of the ceramic substrate 4. RF energy applied to this configuration results in a plasma discharge within the waveguide. It has been discovered that the plasma formed in the intersection regions 12 of the waveguide channels is characterized by a substantially higher current flow compared to the normal waveguide region resulting in a relatively hotter and more intense plasma in this region. This non-uniform gas discharge condition results in a decrease in laser conversion efficiency and in some cases sputtering of the electrode in this region.
Turning to prior art FIG. 4, a conventional waveguide Z-fold resonator configuration which incorporates a U-bore waveguide slab is shown. The phrase "Z-fold" refers to the arrangement of the waveguide channels 6 in a Z pattern (i.e., the three waveguide channels, each passing across the waveguide). Reflecting mirrors 11 are positioned adjacent to channels 6. The output laser beam is emitted through a transmitting mirror 13.
Referring to prior art FIG. 5, an end view of waveguide channels used in conventional waveguides is shown. As is shown, conventional waveguide channels have circular, square and U-shaped cross sections. Each of the channels has an aspect ratio (ratio of height to width) of approximately one-to-one.
Mirrors 11 and 13 (FIG. 4) positioned at the end of each waveguide channel are mounted on optical mounts that must satisfy a number of simultaneous requirements. First, the angular alignment of the mirror must be accomplished without compromising the vacuum integrity of the gas envelope, and second, the alignment should be stable over a wide range of environmental conditions. Additionally, in higher power lasers, the mount should remove excess heat from the mirror's optic substrate to minimize potential damage and surface figure distortions which, if left uncorrected, will lead to a loss of performance and reduced reliability. Fastening the resonator optic to the mount while maintaining angular stability and maintaining a low thermal resistance without distorting the surface figure is also critical, yet difficult to achieve. Finally, to be commercially useful, the cost must be low enough to make economic sense for the application and market being addressed.
Referring to prior art FIG. 6, a cross-sectional view of a conventional gas laser resonator transmitting mirror mount which utilizes a metal post 14 having a transmitting mirror 16, is shown. A flexure arrangement 17 about the flexing point effects an angular movement through the vacuum envelope. The mirror mount is bolted onto a laser housing (not shown) by mounting bolts 19 as is well known. A hermetic seal with the mirror and the laser housing is obtained by "o" rings 21 and 23, respectively. Angular movement of the mount is accomplished through the use of fine threaded adjustment screws 18 located outside the vacuum envelope. The screws in turn apply an angular force to post 14 which is usually monolithic with and hermetically sealed to the laser housing (not shown), as described above. In many applications, four adjusting screws 18 induce orthogonal angular movement but are not as stable as a three point mounting system. The transmitting mirror is held in compression against "o" ring 21 by a press-on cap 20.
The conventional method used to attach mirror 16 to post 14 has disadvantages. Transmitting mirror 16 (and high reflecting mirrors, not shown) is typically attached using press on cap 20 which applies an axial force to the mirror. For cooling purposes, firm intimate contact of the back side of the high reflection mirrors is required and shown in FIG. 7. The placement of press-on cap 20 creates forces in the axial direction of mirror 16 (as shown by the arrows F). This force often results in a deformation in a region 22 of the optic, thus ruining the mirror's surface figure. One approach to circumvent this problem is to mount the mirror against a classic three point contact on the end of the post. This approach, however, compromises the thermal aspects of the design.